Arrangment for operating a steering system for motor vehicle

ABSTRACT

The invention relates to a steering system for a motor vehicle having at least one steerable wheel, an actuator and a superposing gear assembly. The steering movement, which is initiated by the driver of the vehicle, and the movement, which is initiated by the actuator, are superposed by the superposing gear assembly for generating a steering movement of the steerable wheel within pregiven steering stops. The essence of the invention is that the movement, which is initiated by the actuator, is at least reduced before reaching the steering stop. According to the invention, one arrives at a superposed steering with which the steering stops are considered so as to provide increased safety and increased comfort.

STATE OF THE ART

[0001] The invention relates to a method and an arrangement foroperating a steering system for a motor vehicle having the features ofthe preamble of the independent claims.

[0002] Such a steering system is shown in DE-OS 40 31 316 (correspondsto U.S. Pat. No. 5,205,371) and will be explained with respect to FIGS.1 and 2 insofar as relevant for an understanding of the presentinvention. In a steering system of this kind, the steering movements,which are developed by the driver via the steering wheel 11 or 21 (thatis, the steering wheel angle δ_(L), which is detected by the sensor 28),are superposed in the superposing gear assembly 12 or 22 with themovements of the actuator 13 or 23 (that is, the motor angle δ_(M)). Thesuperposed movement δ_(L)′, which so arises, is transmitted via thesteering transmission 14 (or the steering rod 16) to the wheels 15 a and15 b, which are configured so as to be steerable, to adjust the steeringangle δ_(V). Here, the actuator (13 or 23) is configured as an electricmotor. The principal of operation of such a servo steering systemcomprises that the steering can be made very indirect via thetransmission ratio i_(u) of the superposing gear assembly 12 or 22 andthereby attain small steering wheel torques M_(L). Very large steeringwheel angles δ_(L), which are caused in this way, are avoided in thatsuitable motor angles δ_(M) are superposed so that, in correspondence tothe relationship

δ_(L)′=δ_(L)/i_(u)+δ_(M),

[0003] output angles δ_(L)′ can be adjusted required with conventionalsize steering wheel angles. The motor angle δ_(M), which is required tosupport steering, or its desired value, is determined from the steeringwheel angle δ_(L). Furthermore, the motor angle δ_(M) can also beselected in dependence upon signals Sm. These signals Sm represent thevehicle movements detected by the sensors 26.

[0004] Furthermore, it is generally known that the steering wheel anglesδ_(V) are limited by so-called steering stops depending uponconstruction.

[0005] The object of the present invention is to consider the steeringstops for such a superposing steering with respect to increased safetyand comfort.

[0006] This object is solved by the combination of features of theindependent claims.

ADVANTAGES OF THE INVENTION

[0007] As mentioned, the invention relates to a steering system for amotor vehicle having at least one steerable wheel, an actuator and asuperposing gear assembly. By means of the superposing gear assembly,the steering movement initiated by the driver of the vehicle and themovement, which is initiated by the actuator, for generating thesteering movement of the steerable wheel is superposed within pregivensteering stops.

[0008] The essence of the invention is to reduce the movement, which isinitiated by the actuator, before reaching a steering stop.

[0009] According to the invention, no further additional angle can besuperposed when reaching the stops delimiting the steering wheel angles.Otherwise, the steering wheel is rotated against the steering wheelrotational direction which was present previously whereby the driver isgreatly irritated. With the superposition of the additional angle up toclose to the stops, the above-mentioned problem of the steering wheelreturn rotation, however, occurs which is effectively prevented by therun-out of the invention of the steering wheel support in the region ofthe steering stops. This superposition of the additional angle takesplace especially with rapid steering movements such as when turning inthree strokes because of the after-running of the actuator caused byinertia (for example, the after-running of the motor armature when usingan electric motor). With the invention and with a power steeringpursuant to the movement superposition principal, one obtains addedsafety because the driver is not irritated by an unexpected rotation ofthe steering wheel. Furthermore, the application of the invention meansadded comfort because of the continuous run-out of the steering support.

[0010] In an advantageous configuration of the invention, the movement,which is initiated by the actuator, is reduced to the value zero beforereaching a steering stop (that is, up to the standstill of the positiondrive). In the region of the steering stops therefore no servo supporttakes place.

[0011] Furthermore, it can be provided that the movement, which isinitiated by the actuator, is reduced before reaching a steering stop bymeans of a pregivable or pregiven characteristic, especially to thevalue zero.

[0012] It is especially advantageous to reduce the movement, which isinitiated by the actuator, within a predetermined region before reachinga steering stop and especially to reduce this movement to the valuezero. This reduction can also take place with a pregiven or pregivablecharacteristic.

[0013] Furthermore, it is very advantageous that, after the reduction ofthe movement (which is initiated by the actuator), the actuator isswitched off before reaching a steering stop and/or is braked. For thispurpose, it can be especially provided that the switchoff and/or brakingis done in dependence upon a comparison of the detected steeringmovement of the steerable wheel and/or of the detected steeringmovement, which is initiated by the driver of the vehicle, to apregivable or pregiven threshold value. In this configuration, atwo-stage procedure is suggested in the region of the steering stops.First, in a range forward of reaching a steering stop, a reduction ofthe actuator movements takes place and, thereafter, a switchoff and/orbraking is actuated. This has the background that, for the control ofthe actuator, control deviations and/or a certain after-run can occur sothat a simple driving of the actuator in the sense of a reduction is notsufficient in order to bring it to standstill when reaching a steeringstop.

[0014] Reaching the steering stops and/or the start and/or the end ofthe reduction of the movement, which is initiated by the actuator, cantake place with a detection of the steering movement of the steerablewheel and/or via a detection of the steering movement which is initiatedby the driver of the vehicle.

[0015] Furthermore, it is especially provided that the above-mentionedrange is determined by at least one threshold value for the steeringmovement of the steerable wheel and/or for the steering movementinitiated by the driver of the vehicle.

[0016] Additional advantageous embodiments can be seen from thedependent claims.

DRAWINGS

[0017]FIGS. 1 and 2 schematically show the steering system of the stateof the art from which the invention proceeds. FIGS. 3, 4 and 5 showdifferent traces of the steering angle or the speeds thereof.

EMBODIMENT

[0018] In the following, the invention is described with respect to anembodiment. As an example, the description proceeds from a superposingsteering which was initially mentioned.

[0019]FIGS. 1 and 2 show a steering wheel, which can be actuated by thedriver of a vehicle, with reference numerals 11 and 21, respectively.With the actuation of the steering wheel 11, the steering wheel angleδ_(L) is supplied to the superposing gear assembly 12 and, with theactuation of the steering wheel 21, the steering wheel torque M_(L) issupplied to the superposing gear assembly 22. At the same time, themotor angle δ_(M) of the actuator 13 is supplied to the superposing gearassembly 12 and the motor angle δ_(M) of the actuator 23 is supplied tothe superposing gear assembly 22. The actuator can be configured as anelectric motor. At the output end of the superposing gear assembly 12,the superposed movement δ_(L)′ is supplied to the steering transmission14. At the output end of the superposing gear assembly 22, thesuperposed movement δ_(L)′ is supplied to the steering transmission 24.The steering transmission (14 or 24), in turn, charges the steerablewheels 15 a and 15 b with the steering angle δ_(V) via the steering rod16 in correspondence to the total angle δ_(L)′. In FIG. 2, sensors 28and 26 can be seen and sensor 28 detects the steering wheel angle δ_(L)and supplies the same to the control apparatus 26; whereas, the sensorsidentified by reference numeral 26 sense the movements of the vehicle 26(for example, yaw movements, transverse acceleration, road speed, etcetera) and supply the same to the control apparatus 27. The controlapparatus 27 determines a positioning quantity u for driving theactuator (13 or 23). The control apparatus 27 determines the positioningquantity u in dependence upon the detected steering wheel angle δ_(L)and, if required, in dependence upon the vehicle movements.

[0020] As already mentioned, when reaching the stops delimiting thewheel angles δ_(V), no further additional angle δ_(M) may be superposed.Otherwise, the steering wheel (11 or 21) is rotated in opposition to thesteering wheel rotation direction present up to that time whereby thedriver is greatly irritated. For the superposition of the additionalangle δ_(M) up to close to the stops, the above-mentioned problem of thesteering wheel return rotation occurs which can be prevented with therun-out of the steering wheel support in the region of the stop asexplained below. The above-mentioned problem occurs for rapid rotationalmovements (such as when turning in three strokes) because of theafter-running of the actuator (13 or 23) caused by inertia (for example,because of the inertia of the motor armature in the design of theactuator as an electric motor).

[0021] Run-out of the steering support then comprises two parts, therun-out zone and the switchoff threshold. Both are described below.

Run-out Zone

[0022] In a power steering system in accordance with the movementsuperposition principal, a steering ratio

i _(Lenk) =i _(U) *i _(L)

[0023] is determined wherein i_(U) is the transmission ratio of thesuperposing gear assembly (12 or 22) and i_(L) is the transmission ratioof the steering transmission (14 or 24). The steering transmission ratioi_(Lenk) is effective when no additional angles δ_(M) are introduced.With the angle superposition, the angle relationship between δ_(L) andδ_(L)′ or δ_(V) is so changed that the transmission ratio i_(eff)results for the driver.

[0024] According to the invention, the actuator motor (13 or 23) forgenerating the additional angle δ_(M) should be stationary in the regionof the steering stops, that is, the transmission ratio i_(Lenk) shouldbe effective between the steering wheel and wheels in the angular regionjust forward of the steering stops. In the remaining steering anglerange, the angle relationship i_(eff) should apply, however, which is ingeneral clearly less than i_(Lenk).

[0025] The traces shown in FIG. 3 are exemplary and are of the motorangle δ_(M) (trace 4), the wheel steering angle δ_(V) (trace 2) as wellas the trace of the slope 1/i_(eff) (trace 1) and the slope 1/i_(Lenk)(trace 3). The steering wheel angle δ_(L), which is initiated by thedriver, is plotted on the horizontal axis and the wheel steering angleδ_(V) is plotted on the vertical axis and, for the trace 4, the motorangle δ_(M) is plotted on the vertical axis. Furthermore, the run-outregion is marked with the limits A and B and the steering stop is markedwith the limit C.

[0026] To achieve a comfortable steering performance, a continuoustransition from i_(eff) to i_(Lenk) and back must be achieved whichtakes place within a run-out zone (region between limits A and B) whichlies shortly forward of the steering stops (limit C).

[0027] The run-out according to the invention is characterized, forexample, by a parabola segment having a start which exhibits a slopecorresponding to 1/i_(eff) and whose end exhibits a slope correspondingto 1/i_(Lenk).

[0028] As an alternative to the parabola-shaped run-out, the transitionbetween the two slopes in the run-out zone can also be effected with acircular-arc segment or a sine-shaped segment.

[0029] The run-out of the additional steering angle δ_(M) according tothe invention is achieved via a corresponding drive of the actuator 23by the control apparatus 27. This drive can, for example, take place insuch a manner that a desired value δ_(M,des) is supplied to a lowerorder position controller. The position controller then drives the motor23 in such a manner that a corresponding actual angle value δ_(M,act) isadjusted as precisely as possible. The desired value δ_(M,des)represents the desired trace of the motor angle δ_(M).

[0030]FIG. 4 shows the steering wheel angle δ_(L) (horizontal axis)plotted against the steering angle speed dδ_(L)/dt with superposition ofthe motor steering angle δ_(M) (trace 1), the wheel steering angle speeddδ_(V)/dt (trace 2), the motor angle speed dδ_(M)/dt (trace 3) and thewheel steering angle speed dδ_(V)/dt without superposition of the motorsteering angle δ_(M) (trace 4). The run-out zone is again characterizedby the limits A and B and the steering stop is characterized with thelimit C.

[0031] With respect to FIG. 4, one recognizes that the parabola-shapedrun-out is favorable with respect to the trace 3 of the motor anglespeed dδ_(M)/dt because this speed is continuously reduced in therun-out zone (A, B).

Switchoff Threshold

[0032]FIG. 5 shows a similar plot as FIG. 3.

[0033] The actuator 13 or 23 for the additional angle δ_(M) isconventionally a position controlled motor, such as an electric motor.Here, control deviations (δ_(M,act)−δ_(M,des)) occur from the pregivenangle desired value δ_(M,des), for example, for rapid steering maneuversor even via an after-running of the motor armature and the connectedtransmission parts because of the mass moment of inertia. It thereforeoccurs that the motor still rotates above the run-out zone (over thelimit B to the steering stop) even though it must be at standstillalready in this region.

[0034] As can be seen in FIG. 5, for this reason, a switchoff thresholdD is fixed shortly after the end B of the run-out zone forward of thesteering stop C. When this switchoff threshold D is exceeded, theactuator is braked to standstill by a corresponding supply of currentand is then switched off.

[0035] The gear assembly 12 or 22, which is generally configured as aself-restraining gear assembly, prevents a change of the additionalangle δ_(M,act) via possibly applied torques whereby a return rotationof the steering wheel 11 or 21 is reliably avoided.

1. Method for operating a steering system for a motor vehicle having atleast one steerable wheel (15 a, 15 b), an actuator (13; 23) and asuperposing gear assembly (12; 22); the steering movement δ_(L), whichis initiated by the driver of the vehicle, and the movement δ_(M), whichis initiated by the actuator (13; 23), are superposed by the superposinggear assembly for generating the steering movement δ_(V) of thesteerable wheel (15 a, 15 b) within pregiven steering stops C,characterized in that the movement δ_(M), which is initiated by theactuator (13; 23), is at least reduced before reaching a steering stopC.
 2. Method of claim 1 , characterized in that the movement δ_(M),which is initiated by the actuator (13; 23), is reduced to the valuezero before reaching a steering stop C.
 3. Method of claim 1 ,characterized in that the movement δ_(M), which is initiated by theactuator (13; 23), is reduced with a pregivable or pregivencharacteristic before reaching a steering stop C.
 4. Method of claim 1 ,characterized in that the movement δ_(M), which is initiated by theactuator (13; 23), is at least reduced within a predetermined range (A,B) before reaching a steering stop and is reduced especially with apregivable or a pregiven characteristic.
 5. Method of claim 4 ,characterized in that the actuator is switched off and/or is brakedbefore reaching a steering stop after the reduction of the movementδ_(M), which is initiated by the actuator (13; 23); and, it isespecially provided that the switchoff and/or braking is dependent upona comparison of the detected steering movement δ_(V) of the steerablewheel (15 a, 15 b) and/or of the steering movement δ_(L), which isinitiated by the driver of the vehicle, to a pregivable or a pregiventhreshold value D.
 6. Method of one of the above claims, characterizedin that the reaching of the steering stops and/or the begin and/or theend of the reduction of the movement δ_(M), which is initiated by theactuator (13; 23), takes place via detection of the steering movementδ_(V) of the steerable wheel (15 a, 15 b) and/or by a detection of thesteering movement δ_(L), which is initiated by the driver.
 7. Method ofclaims 4 and 6, characterized in that the range (A, B) is determined byat least a threshold value for the steering movement δ_(V) of thesteerable wheel (15 a, 15 b) and/or for the steering movement δ_(L),which is initiated by the driver of the vehicle.
 8. Arrangement foroperating a steering system for a motor vehicle having at least onesteerable wheel (15 a, 15 b), an actuator (13; 23) and a superposinggear assembly (12; 22); the steering movement δ_(L), which is initiatedby the driver of the vehicle, and the movement δ_(M), which is initiatedby the actuator (13; 23) are superposed by the superposing gear assemblyfor generating the steering movement δ_(V) of the steerable wheel (15 a,15 b) within pregiven steering stops C, characterized in that means 27are provided by means of which the movement δ_(M), which is initiated bythe actuator (13; 23), is at least reduced before reaching a steeringstop C, especially to the value zero.
 9. Arrangement of claim 8 ,characterized in that the means 27 is configured such that the movementδ_(M), which is initiated by the actuator (13; 23), is reduced with apregivable or pregiven characteristic before reaching a steering stop C.10. Arrangement of claim 8 , characterized in that means are providedwhich cause the actuator to be switched off and/or be braked beforereaching a steering stop after the reduction of the movement δ_(M),which is initiated by the actuator (13; 23); and, it is especiallyprovided that the switchoff and/or braking is dependent upon acomparison of the detected steering movement δ_(V) of the steerablewheel (15 a, 15 b) and/or of the detected steering movement δ_(L), whichis initiated by the driver of the vehicle, to a pregivable or a pregiventhreshold value D.